cmd/compile: add missing method info for method with correct name except for case

When being used by the compiler, augment the types2 missing method
message with extra info, if a method is missing, but a method with the
correct name except for case (i.e. equal via string.EqualFold()) is
present. In that case, print out the wanted method and the method that
is present (that has the wrong case).

In the 1.17 compiler, we don't do this case-folding check when assigning
an interface to an interface, so I didn't add that check, but we could
add that.

Fixes #48471

Change-Id: Ic54549c1f66297c9221d979d49c1daa719aa66cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/363437


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>